Polypeptide factors that bind to cell surface receptors are powerful mediators of cell-cell communication and have important roles in physiology and disease. The existence of a large number of unidentified polypeptide factors is implied by the identification of "orphan receptors" which appear to be cell surface receptors but for which the ligands are unknown. Identification of these ligands is a critical step in understanding the biology of the orphan receptors, and will also provide important advances in our understanding of cell-cell interaction in general. Moreover, many of the ligands will be good candidates for therapeutic use. We recently identified the ligand (KL) of the orphan receptor tyrosine kinase encoded by the c-kit proto-oncogene by a generally applicable approach: the receptor extracellular domain was genetically fused to placental alkaline phosphatase, producing a soluble receptor affinity reagent with an enzyme tag that could be easily and sensitively traced. The major objective of the present proposal is to develop our soluble receptor affinity approach further, and apply it to identify the ligand of an orphan receptor protein tyrosine phosphatase. Although orphan receptor tyrosine phosphatases are being identified at a rapid rate, little information is available on any of their ligands. Nonetheless, the involvement of the receptors in the control of tyrosine phosphorylation indicates that the ligands are likely to have potent biological effects. In addition to our work on phosphatases, we will perform experiments on the interaction of the c-kit receptor and its recently identified ligand KL. These experiments will have two goals. The first will be to develop methods that can be applied to receptor tyrosine phosphatases and other orphan receptors. The second goal will be to concurrently derive useful information on the biology of c-kit and KL. Studies with the soluble receptor fusion protein are designed to provide important new information on the distribution and the isoforms of the KL polypeptide that are present in tissues. We will also carry out a structure / function analysis of the interaction of c-kit and KL. This will allow us to map functional domains and also to investigate the functional interrelationships between ligand-receptor binding, cell-cell adhesion, proliferation and migration, all of which may be mediated by KL.